The present invention relates to an apparatus for handling nuclear fuel assemblies, particularly in a fast neutron nuclear reactor and to an assembly for use with such an apparatus.
The assemblies constituting the core of a fast neutron nuclear reactor conventionally comprise elongated boxes or cases containing a bundle of sheathed rods containing the fissile or fertile material. For simplification reasons, throughout the present application, these two types of assembly are referred to as "nuclear fuel assemblies".
In order to permit the handling thereof in the actual reactor, in storage cells attached thereto, or even during transportation or subsequent handling of these assemblies, the boxes have at their upper end a gripping head permitting the raising and transfer of each assembly with the aid of an appropriate handling apparatus.
With respect to the actual reactor, it generally comprises a vertically axed vessel sealed by a horizontal slab at its upper end and filled with a liquid cooling metal such as sodium, surmounted by a layer of a neutral gas such as argon. The vessel also contains the reactor core constituted by a group of a certain number of nuclear fuel assemblies arranged in accordance with a hexagonal network or lattice. Each assembly has a leg, which engages in a supporting member resting on the bottom of the vessel. The loading and unloading of the assemblies generally take place by means of a handling container moving within a ramp linking the interior of the vessel with an external chamber generally positioned above the slab.
In existing fast neutron reactors, such as Phenix or Super-Phenix reactors, the handling of the fuels between the core and the handling container is carried out by means of a handling poker and/or a handling arm suspended on rotary plugs disposed in the slab above the reactor core and whose axes are displaced in such a way that the combined rotation of these plugs makes it possible to have access to all the assemblies constituting the core.
These pokers or arms are equipped with handling means incorporating articulated gripping jaws or claws, whose opening and closing are controlled by a rigid rod passing through the slab. When an assembly is extracted from the core or has to be introduced into the latter, the control rod rises above the slab by a height corresponding to the interior of a tight enclosure provided for this purpose above the slab. For example, that part of the assembly handling means above the slab has a height of 7 meters in the Phenix reactor and 14 meters in the Super-Phenix reactor.
The above description clearly shows that the presently used handling means have considerable overall dimensions and consequently a high cost. Moreover, the need for a perfect sealing when they pass through the slab makes the maintenance and emergency repair operations relatively complex.
Moreover, the available space for the poker in the small rotary plug is not very great, bearing in mind that the latter is traversed by the mechanisms ensuring the control of the control rods used for controlling and stopping the reactor. Thus, the overall diameter of the poker must be as small as possible.
The design of the handling means must also take account of a certain number of constraints due to the modification of the structure of the assemblies resulting from their irradiation in the core. Thus, this irradiation leads to a displacement of the network or lattice formed by the assemblies. In the same way, the irradiated assemblies are frequently curved and have a certain elongation. Moreover, the handling means must make it possible to orient the assembly by .+-.180.degree..
Finally and independently of all the constraints referred to hereinbefore, the assembly handling means must have an absolute security of gripping, in order that in no case the assembly can be dropped during handling.